New Clues to Compositions of TRAPPIST-1 Planets

The seven Earth-size planets of TRAPPIST-1 are
all mostly made of rock, with some having the potential to hold more water than
Earth, according to a new study published in the journal Astronomy and
Astrophysics. The planets' densities, now known much more precisely than
before, suggest that some planets could have up to 5 percent of their mass in
water -- which is 250 times more than the
oceans on Earth.

The form that water would take on TRAPPIST-1 planets
would depend on the amount of heat they receive from their star, which is a
mere 9 percent as massive as our Sun. Planets closest to the star are more
likely to host water in the form of atmospheric vapor, while those farther away
may have water frozen on their surfaces as ice. TRAPPIST-1e is the rockiest
planet of them all, but still is believed to have the potential to host some
liquid water.

Astronomers using the Hubble Space Telescope have conducted the first spectroscopic survey of Earth-sized planets in the TRAPPIST-1 system's habitable zone. Hubble reveals that at least the inner five planets do not seem to contain puffy, hydrogen-rich atmospheres similar to gaseous planets such as Neptune. This means the atmospheres may be more shallow and rich in heavier gases like carbon dioxide, methane, and oxygen.

"We now know more about TRAPPIST-1 than any other
planetary system apart from our own," said Sean Carey, manager of the
Spitzer Science Center at Caltech/IPAC in Pasadena, California, and co-author
of the new study. "The improved densities in our study dramatically refine
our understanding of the nature of these mysterious worlds."

Since the extent of the system was revealed in February
2017, researchers have been working hard to better characterize these planets
and collect more information about them. The new study offers better estimates
than ever for the planets' densities.

What is TRAPPIST-1?

TRAPPIST-1 is
named for the Transiting Planets and Planetesimals Small Telescope
(TRAPPIST) in Chile, which discovered two of the seven planets we know of today
-- announced in 2016. NASA's Spitzer Space Telescope, in collaboration with
ground-based telescopes, confirmed these planets and uncovered the other five
in the system.

Since then, NASA's Kepler space telescope has also observed
the TRAPPIST-1 system, and Spitzer began a program of 500 additional hours of TRAPPIST-1
observations, which will conclude in March. This new body of data helped study
authors paint a clearer picture of the system than ever before -- although
there is still much more to learn about TRAPPIST-1.

The TRAPPIST-1 planets huddle so close to one another that a
person standing on the surface of one of these worlds would have a spectacular
view of the neighboring planets in the sky. Those planets would sometimes
appear larger than the Moon looks to an observer on Earth. They may also be
tidally locked, meaning the same side of the planet is always facing the star, with
each side in perpetual day or night. Although the planets are all closer to
their star than Mercury is to the Sun, TRAPPIST-1 is such a cool star, some of
its planets could still, in theory, hold liquid water.

In the new study, scientists led by Simon Grimm at the University
of Bern in Switzerland created computer models to better simulate the planets
based on all available information. For each planet, researchers had to come up
with a model based on the newly measured masses, the orbital periods and a
variety of other factors -- making it an extremely difficult, "35-dimensional
problem," Grimm said. It took most of 2017 to invent new techniques and run
simulations to characterize the planets' compositions.

This chart shows, on the top row, artist concepts of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii, masses, densities and surface gravity as compared to those of Earth. Credit: NASA/JPL-Caltech
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What might these planets look like?

It is impossible to know exactly how each planet looks,
because they are so far away. In our own solar system, the Moon and Mars have
nearly the same density, yet their surfaces appear entirely different.

"Densities,
while important clues to the planets' compositions, do not say anything about
habitability. However, our study is an important step forward as we continue to
explore whether these planets could support life," said Brice-Olivier Demory, co-author at the
University of Bern.

Based on available data, here are scientists' best
guesses about the appearances of the planets:

TRAPPIST-1b, the innermost planet, is likely to have a
rocky core, surrounded by an atmosphere much thicker than Earth's. TRAPPIST-1c also
likely has a rocky interior, but with a thinner atmosphere than planet b. TRAPPIST-1d
is the lightest of the planets -- about 30 percent the mass of Earth.
Scientists are uncertain whether it has a large atmosphere, an ocean or an ice
layer -- all three of these would give the planet an "envelope" of
volatile substances, which would make sense for a planet of its density.

Scientists were surprised that TRAPPIST-1e is the only
planet in the system slightly denser than Earth, suggesting it may have a
denser iron core than our home planet. Like TRAPPIST-1c, it does not
necessarily have a thick atmosphere, ocean or ice layer -- making these two
planets distinct in the system. It is mysterious why TRAPPIST-1e has a much
rockier composition than the rest of the planets. In terms of size, density and
the amount of radiation it receives from its star, this is the most similar
planet to Earth.

TRAPPIST-1f, g and h are far enough from the host star
that water could be frozen as ice across these surfaces. If they have thin
atmospheres, they would be unlikely to contain the heavy molecules of Earth,
such as carbon dioxide.

"It is
interesting that the densest planets are not the ones that are the closest to
the star, and that the colder planets cannot harbor thick atmospheres," said
Caroline Dorn, study co-author based at the University of Zurich, Switzerland.

This graph presents known properties of the seven TRAPPIST-1 exoplanets (labeled b thorugh h), showing how they stack up to the inner rocky worlds in our own solar system. Credit: NASA/JPL-Caltech
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How do we know?

Scientists are able to calculate the densities of the
planets because they happen to be lined up such that when they pass in front of
their star, our Earth- and space-based telescopes can detect a dimming of its
light. This is called a transit. The amount by which the starlight dims is
related to the radius of the planet.

To get the density, scientists take advantage of what are
called "transit timing variations." If there were no other
gravitational forces on a transiting planet, it would always cross in front of its
host star in the same amount of time -- for example, Earth orbits the Sun every
365 days, which is how we define one year. But because the TRAPPIST-1 planets
are packed so close together, they change the timing of each other's "years"
ever so slightly. Those variations in orbital timing are used to estimate the
planets' masses. Then, mass and radius are used to calculate density.

This illustration shows the seven Earth-size planets of TRAPPIST-1. The image does not show the planets' orbits to scale, but highlights possibilities for how the surfaces of these intriguing worlds might look. Image Credit: NASA/JPL-Caltech
› Full image and caption

Next
Steps

The next step
in exploring TRAPPIST-1 will be NASA's James Webb Space Telescope, which will
be able to delve into the question of whether these planets have atmospheres
and, if so, what those atmospheres are like. A recent study using NASA's Hubble Space Telescope found no
detection of hydrogen-dominated atmospheres on planets TRAPPIST-1d, e and f --
another piece of evidence for rocky composition -- although the
hydrogen-dominated atmosphere cannot be ruled out for g.

Illustrations
of these worlds will change as ongoing scientificinvestigations home in
on their properties.

"Our conceptions of what these planets look like today
may change dramatically over time," said Robert Hurt, senior visualization
scientist at the Spitzer Science Center. "As we learn more about these
planets, the pictures we make will evolve in response to our improved
understanding.